◆ Local transmission of dengue, Japan September 5, 2014 (as of 1100 hours)

Dengue fever is an acute febrile illness caused by a mosquito-borne viral infection. Dengue is found widely throughout the world, such as in Asia, Central and South America, and Africa. The virus is transmitted via mosquitoes, but is not directly transmitted person-to-person. The mosquito Aedes albopictus(active mostly during daylight hours in outdoor settings) is a vector competent in transmitting dengue virus, present in much of Japan (detected in all but the northernmost Hokkaido island and the most northern prefecture, Aomori, on the main island Honshu). Infection with dengue virus may cause such symptoms as high fever, headache, muscle pain, and rash, but may also be mild or asymptomatic. Signs and symptoms may last around a week for the majority of dengue fever cases, but a proportion may progress to more severe forms, such as dengue hemorrhagic fever. In recent years, approximately 200 imported cases of dengue (those that had onset after returning to Japan following overseas travel) have been detected and treated annually, reported through national surveillance 1, 2, 3). Domestic transmission of dengue virus was present in Japan during the 1940s 4, 5), but a domestic case had not been confirmed since then. However, in 2013, a travel-associated dengue case in Germany was suspected to have been infected with dengue in Japan in 20136), and since then, the country has been preparing guidelines and relevant documents to guide response activities should local transmission occur.

On August 27 and 28, 2014, three autochthonous dengue cases were confirmed. Since then, as of September 5, 2014, 1100 hours (Ministry of Health, Labour and Welfare (MHLW), Japan), a total of 66 autochthonous cases of dengue fever have been confirmed. None of these cases had recent overseas travel history. While cases were reported from throughout Japan (residents in Hokkaido, Aomori, Ibaraki, Gunma, Saitama, Chiba, Tokyo, Kanagawa, Niigata, Yamanashi, Osaka, and Ehime prefectures), all 66 cases were associated with visiting Yoyogi Park, a large park (or its vicinity) in metropolitan Tokyo.

There is no specific medicine or treatment for dengue fever, and treatment is supportive. However, the proportion of infections that result in severe forms are rare, and after an incubation period of 3-7 days after being bitten by an infected mosquito, majority of those who develop symptoms recover in about a week. To prevent dengue infection, it is important to avoid bites from Aedes albopictus, active mostly during daylight hours in outdoor settings. Reducing exposed areas of skin when outdoors by wearing long sleeves and pants and wearing insect repellent with DEET are effective measures. Furthermore, in addition to keeping mosquitoes from entering buildings, it is important to eliminate places where mosquitoes lay their eggs (and where their larva thrive), such as artificial containers and watering containers or dishes.

The Ministry of Health, Labour and Welfare is actively providing information regarding its prevention and response activities on its website 7) and coordinating with local government counterparts in information collection and assessment. The Tokyo Metropolitan government is actively conducting vector control activities, and enhancing risk communication activities 8, 9, 10). Since 2004, the Tokyo Metropolitan government has been testing mosquitoes captured from 16 collection sites in the Tokyo metropolitan area, and had not detected dengue virus. However, on September 4, 2014, the Tokyo Metropolitan government announced that dengue virus was detected from mosquitoes collected from Yoyogi Park 8, 10). The Tokyo Metropolitan government is planning to increase collection sites and enhance vector surveillance. The National Institute of Infectious Diseases, in coordination with its partners, is providing technical support regarding virus testing and vector control, and enhancing dissemination of information based on surveillance data.